But it can't have been friendly, as the dog's owner heard a 'scream, a roar and a commotion' from the back garden.

George Watson, 43, had been having a bath and ran outside in his towel, to see the thief zooming off down the driveway in his van.

Apart from a very agitated dog, the only evidence the man left behind was the torn Tshirt.

Mr Watson, who lives with his wife Lesley, 43, and daughter Molly, ten, in Gordon, Berwickshire, assumes Cromwell tore the shirt from the thief's back to stop him stealing the family's lawnmower, which had been taken from the shed and left by the gate.

Cromwell stands guard: Owner George Watson with son Christopher and daughter Molly

At just under 22 stone the dog could be the world's heaviest - so it is lucky that this is all he tore off.

'By the time Cromwell had finished with it it was covered in holes,' his owner said.

Mr Watson, a regional manager of an IT company, does not expect the man to return for another go.

If the burglar decided to make a complaint, Mr Watson could not be prosecuted under the Dangerous Dogs Act because the incident took place on his private property.

The last dog to hold the title of world's heaviest was Hercules, an English mastiff from the U.S., which weighed 282lb (just over 20 stone) until its recent death.

The Mastiff's Tale

The breed now known as the English mastiff was brought to this country by Phoenician traders as early as the 6th century BC.

After the Roman invasion, they were used to fight in arenas against other large animals such as lions.

By the end of the Second World War the breed was almost extinct in the UK but was re-established with the help of imports.

Mastiffs are the heaviest dogs in the world and the normal weight for a fully-grown adult is about 14 stone.

Although bred as guard dogs, English mastiffs make very good family pets. They are good with children, rarely bark and tend to defend rather than attack.